The present invention pertains to systems for measuring and indicating the depth of material in a container.
There are many instances in which it is desirable or necessary to provide a quick measurement of the amount of material stored in a container. In a typical industrial or agricultural setting, a bin or other container is provided for temporary storage of some material, such as grain, feed, cement, carbon black, etc. Typically, quantities of material are added to and withdrawn from the container, from time to time, and the need arises for knowing the amount of material in the container at any given moment.
Although it is in theory possible to provide actual measurement of quantities of material added to and withdrawn from the container, thereby to maintain a tally of its actual contents, such schemes are generally not practical because of the length of time and the degree of complexity involved in making actual measurements of the material. Rather, it is generally preferable to allow for rapid adding or subtracting of material from the container, by conveyor, dump truck, auger, or the like, without the necessity of actual measurement of the material so added or removed. This in turn implies a need for an indirect means of measurement, and preferably one which is quickly and accurately made.
Since the total volume of the container is known, having once been measured or calculated, the simplest method of measuring material in the container involves measurement of the depth of the material at a given time. Because visual inspection is generally inconvenient or impractical, various systems have been proposed in the prior art for measurement of the depth of the material by means of a sensor or sensors placed within the container, and some type of readout means located externally of the such as an office which contains readouts for numerous containers in a given installation.
Sensors for measuring depth of granular material in containers have been proposed which operate electrically, or by fluid pressure. In either case, an elongated sensor or a plurality of sensors are generally placed vertically in the container so as to be progressively immersed in the material as it is added to the container. One type of prior art electrical sensor uses a capacitance probe and circuitry for measuring changes in capacitance as the container fills with material. Unfortunately, such probe often cannot distinguish between solid material or dust settling on the probe, and additionally are subject to stability and calibration problems.
Another type of electrical prior art system has used a resistance probe. The material in the bin is suppose to exert pressure on the probe, causing it to change its resistance characteristics, to the extent of immersion in the material. Another type of prior art electromechanical system involves a series of pressure actuated switches at various depths within the bin. Each switch is operated by a diaphragm which is suppose to be depressed by immersion in the material. A readout device indicating which switches have been thrown indicates the depth of material in the bin.
Prior art fluid type systems have been proposed which involve the use of an elongated sensor mounted vertically in the bin. The sensor has a movable or flexible portion which is intended to be compressed by the accumulating material in the bin so as to displace a quantity of fluid from within the sensor. The fluid bellow or the like is then provided to measure the displaced fluid.
Regardless of whether operated by mechanical or electrical means, prior art systems have suffered from certain problems and inaccuracy due to the amount of force required to actuate or compress the sensor. This is because most dry products develop horizontal forces which are very low compared to the weight per cubic foot of material. This is especially true of very light materials such as carbon black or sawdust, in which the very low horizontal forces approach the limits of the sensitivity of prior art sensors.
It will be appreciated that the prior art sensors depend upon an assumption that the granular material will behave in a quasi-fluid manner, so that the weight or pressure due to the depth of the material will be exerted uniformly in a horizontal, as well as in a vertical direction, as would be the case with a true fluid. Unfortunately, in reality most granular materials behave partially as a solid and only a portion of the total force is distributed horizontally against the sensor.
The weakness of the horizontal forces developed presents a distinct problem in designing a sensor which depends upon the horizontal forces to overcome the inherent preload of the sensor for actuation. The problem is inherently the same, no matter whether the sensor preload is due to the necessity of compressing fluid against a bellows, or the necessity of applying pressure to a resistance probe or overcoming a spring bias in a pressure switch. Often the preload of the sensor has been as great or greater than the weak horizontal forces developed by the material. Needless to say, such a condition will result in inaccurate or inconsistent readings. Further, even though the sensitivity of the sensors in some prior art systems has been made very high, unacceptable results have nonetheless still been obtained, particularly with lightweight granular materials.
The problem is further compounded by the tendency of some materials to cake or bridge in the container--a condition which results in local isolated pockets in which the material behaves entirely as a solid body, with no horizontal forces. Additionally, many dry materials flow freely if held for only short periods of time in a vessel. However, when static storage is involved for an extended length of time, which depends of course on the material involved, they tend to become caked or bridged, in some cases to the extent that they become firm enough for a person to walk on. Materials of this type include powdered coal, clay, cement, flour, carbon black, grain and feed, for example. This caking and bridging is due to a number of diverse causes, including deaeration, chemical changes, presence of moisture, etc. The variability of these factors makes it very difficult to build a sensor and system giving acceptable and repeatable operation.
Because of the complexity and expense involved in building an electrical sensor typically having a great number of switches for installation in a deep container, and because of the potential explosion hazard with electrical systems, it has been recognized that a fluid displacement system would have the advantages of simplicity, safety and lower cost. Unfortunately, these greater theoretic advantages of fluid systems have not been recognized in systems proposed in the prior art because of certain other problems which have not been satisfactorily solved, at least in a feasible and economical manner.
Typical prior art fluid systems use a flexible member or diaphragm defining an air passage, positioned vertically in the container. A fluid communication line from the top of the sensor connects to a bellows, piston, or other displacement indicating device to which is attached an indicating pointer or other readout means. An example of such a system is found in U.S. Pat. No. 3,401,562, issued to W. A. Reaney. As material in the container causes compression of the sensor, the bellows or piston is caused to move in response. Such systems are subject to the major problems of temperature sensitivity and inaccuracies due to leaks in the system, in addition to product bridging and the weakness of the horizontal forces developed in the product as discussed above, upon which such systems must rely for compression of their sensors.
In this type of prior art system, the entire fluid system including the sensor, interconnecting line, and the bellows or other readout device must be sealed from the atmosphere. Unfortunately, this renders the system highly susceptible to erroneous readings caused by temperature changes. When the temperature increases, the air in the system expands, giving erroneous readings, and vice versa when the temperature drops. Of course, use of a liquid instead of a gas as the working fluid would help in this respect, but it is generally not feasible to do so, because the density of the liquid would build up a significant pressure head in the elongated vertical sensor, requiring excessive and unrealistic displacement forces to be supplied by the material.
In order to overcome temperature problems, systems have been proposed in the prior art which include elaborate temperature compensating bellows, as shown in U.S. Pat. No. 3,290,938 issued to R. R. Miller for example. Unfortunately, this proposed solution leads to greater complexity and increased costs, and potentially increases the vulnerability of the system to leaks.
Because the prior art fluid systems depend upon a completely sealed fluid system, the presence of even a minute leak will seriously affect long term accuracy. Although it is possible to build a system relatively free of gross leaks, the extent of the sensor, and the other tubes and devices involved in the measurement system makes it extremely difficult to guard against long term, slow leaks which will degrade accuracy over a period of weeks or months. Generally recalibration in this type of prior art system is not feasible, short of completely emptying or completely filling the container.
The present invention solves these and other problems existing in the art by providing an improved depth measurement system which takes advantage of the inherent simplicity and economies of a fluid system, but which works upon a different principle so as to avoid the problems heretofore existing in the art.